1. Field of the Invention
The present invention relates to a memory device, and is applied, for example, to a memory card including a flash memory.
2. Description of the Related Art
In recent years, there has been an increasing demand for a nonvolatile memory in accordance with an increase in memory capacity thereof. However, in order to increase the memory capacity of the nonvolatile memory, it is necessary to perform high-density recording. As a result, the distance between neighboring memory cells becomes smaller. Consequently, garbling of data occurs, and data in the memory cell is erroneously read out by the influence of neighboring data values. Recovery of data is impossible even if an error correcting code (ECC) with a high error correcting capability is used.
For example, in the case of a single-level NAND flash memory which is capable of recording 1-bit data in one memory cell, carriers are injected in a floating gate (FG) when data “0” is written in a memory cell. However, with the progress in microfabrication for increasing the recording density, the distance between memory cells decreases and the value of capacitive coupling occurring between the floating gates (FG) of neighboring memory cells increases. In this case, if carriers are injected in the floating gate (FG) when data “0” is written in the memory cell, the potential of the floating gate of a neighboring memory cell varies due to capacitive coupling. There is the tendency that owing to this variation in potential, garbling of data occurs and data “1” written in the neighboring memory cell is erroneously read out as data “0”, resulting in irrecoverability of data and degradation in reliability. Moreover, the garbling of data may occur in either a memory cell which neighbors in a bit line direction or a memory cell which neighbors in a word line direction.
Jpn. Pat. Appln. KOKAI Publication No. 11-317095, for instance, discloses a semiconductor memory device which can efficiently and exactly execute error detection and error correction, even if stored multi-value information is lost due to garbling of data resulting from, for example, degradation of a memory cell, which necessarily occurs with repeated use.